Constify ctrl_cmd struct fields where appropriate

Change-Id: I3f55c1d4b965d215dc9b16f4b284b7fea4bde9e9
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 648b6a6..b67d6dd 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -4,3 +4,4 @@
 libosmocore	change major	size of ph_data_param struct changed / Extend L1SAP PH-DATA with presence information
 libosmocore	change major	size of ph_data_param struct changed / Extend L1SAP PH-DATA with measurement information
 libosmocore	change major	size of ph_tch_param struct changed / Extend with RTP Marker
+libosmocore	change minor	mark as const id, variable & value fields of ctrl_cmd struct
diff --git a/include/osmocom/ctrl/control_cmd.h b/include/osmocom/ctrl/control_cmd.h
index 8f2eaa2..cd96038 100644
--- a/include/osmocom/ctrl/control_cmd.h
+++ b/include/osmocom/ctrl/control_cmd.h
@@ -53,10 +53,10 @@
 struct ctrl_cmd {
 	struct ctrl_connection *ccon;
 	enum ctrl_type type;
-	char *id;
+	const char *id;
 	void *node;
-	char *variable;
-	char *value;
+	const char *variable;
+	const char *value;
 	char *reply;
 };